Wide band gap copolymers based on phthalimide: synthesis, characterization, and photovoltaic properties with 3.70% efficiency

Wide band gap polymers (E-g > 2 eV) are essential to polymer solar cells (PSCs) due to their potential applications in tandem solar cells. In this study, three wide band gap polymers, P1, P2, and P3, were synthesized by Stille coupling of the electron-acceptor unit of phthalimide (PhI) and the electron-donor unit of 4,8-bis-(2-ethylhexyloxy), 4,8-bis(n-dodecyloxy) and 4,8-bis(2-(2-ethylhexylthienyl) benzodithiophene (BDT), respectively, and then were physicochemically characterized. Optical tests found that all these three polymers displayed a film absorption peak around 500 nm and their optical band gap is in the range of 2.07-2.13 eV. Electrochemical tests indicated that the three polymers possess deeply layered HOMO energy levels (-5.36 eV to -5.57 eV). P1 is poorly soluble, however P2 and P3 were both applied in PSCs with PC71BM as the electron-acceptor material. The photovoltaic tests indicated that both the polymers exhibited a higher open-circuit voltage (V-oc) of 0.80 V (P2) and 0.89 V (P3) because of their deeper HOMOs than P3HT. Polymer P2 with 4,8-bis(n-dodecyloxy) BDT as the electron-donor unit exhibited poor power conversion efficiency (PCE) of 1.50%, while P3, containing 4,8-bis(2-(2-ethylhexylthienyl) BDT and PhI, exhibited a promising PCE of 3.70%. This significant increase of the PCE is mainly from the nearly 2-fold increase of the short-circuit current density, J(sc) (7.01 mA cm(-2) vs. 3.43 mA cm(-2)) and also from an improvement in both the fill factor, FF (58.6% vs. 54.7%), and Voc (0.89 V vs. 0.80 V). We attribute the promoted photovoltaic performance of P3 with respect to P2 to its broader absorption, deeper HOMO level, weaker molecular aggregation, better miscibility with PC71BM, optimized film morphology, and finally, better hole mobility compared to P2, all of which originated from the structure differences between alkoxyl and alkylthienyl BDT. The PCE for P3 is, to the best of our knowledge, among the top 2 efficiencies for the wide band gap polymers (the highest one is 5.04% reported very recently in Polym. Chem., 2013, 4, 57) and the highest efficiency reported to date for PhI-based polymers. Our results enriched the tool-box for wide band gap polymers with enhanced efficiencies higher than 3.5%. Accordingly, the wide band gap polymer, P3, should be a potential candidate for applications in tandem solar cells.